Async trait methods for no_std

Features like async-trait, avoid using Box and dyn.

This crate is ready for #![no_std] when PR69033 merged.

Thanks to crate async-trait, some code from these.

WARNING: This crate use some unstable even incomplete feature.

Features

• [X] define async fn in trait without default implementation.
• [ ] support default implementations in trait.
• [ ] test more feature support.

Works

• [X] Solve lifetime for self.
• [ ] Find a way to process default implementations in trait.
  • [X] Don't define type alias as associated type.
  • [ ] Error: type parameter Self is part of concrete type but not used in parameter list for the impl Trait type alias

Usage

Please enable feature type_alias_impl_trait and generic_associated_types;

```rust

![feature(typealiasimpl_trait)]

![feature(genericassociatedtypes)]

use asynctraitstatic::async_trait;

async fn hello() -> u8 { 1 }

[async_trait]

trait AsyncFnTrait { async fn run(&self); }

struct AsyncStruct;

[async_trait]

impl AsyncFnTrait for AsyncStruct { async fn run(&self) { hello().await; } }

```